This invention relates to a ferrite magnetic film structure which has a magnetic anisotropy in an optional direction and is capable of continuously changing the direction of magnetic anisotropy, to a method of manufacturing the ferrite magnetic film structure, and to a planar magnetic device employing a ferrite magnetic film structure having a magnetic anisotropy.
In recent years, electronic devices of various kinds are increasing miniaturized, resulting in a trend to manufacture various electronic devices by making use of a thin film process. In this trend, magnetic devices such as an inductor, a transformer and a magnetic head are now developed in the form of planar magnetic devices such as those of shell type structure where a planar coil formed on a substrate is covered with a magnetic film, or those where green sheets are laminated and then baked.
In these planar type magnetic devices, the coil is manufactured by making use of a liquid phase deposition method such as an electrolytic plating method, or a vapor deposition method such as a sputtering method and a vacuum vapor deposition method. On the other hand, a magnetic film is manufactured by making use of vapor deposition method such as a sputtering method and a vacuum vapor deposition method, or a method of coating a dispersed solution of magnetic material in a resin. Alternatively, a magnetic film is manufactured by making use of a film-like coil and a film-like magnetic film to be obtained through quenching of a magnetic material from a high temperature.
In the latter method, the thickness of a thin film magnetic device inevitably becomes as thick as about 1 mm. Moreover, since the electric contact thereof with an external member cannot be achieved in the same manner as that of a semiconductor component, there is a limit in thinning or miniaturizing a device composed of a combination of a thin film magnetic device and another semiconductor component.
On the other hand, the former method is advantageous in the respect that the thinning of a thin film magnetic device can be easily realized and that a thin film magnetic device can be mounted in the same manner as in the case of a semiconductor component. However, the thinning in thickness of a magnetic film to be manufactured by making use of a vapor phase growth method is limited in view of mass productivity and stress. Additionally, the former method is accompanied with a drawback in that, due to the phenomenon that the magnetic film is magnetically saturated, the characteristics such as current capacity are subjected to limitation.
In view of overcoming these problems, there is now adopted, in the formation of a magnetic film, a method of forming a ferrite layer by means of a wet plating method (Japanese Patent Unexamined Publication S/63-268210) or by making use of a resin film (Japanese Patent Unexamined Publication H/1-173702).
Further, since the ratio of core loss due to magnetizing hysteresis becomes higher as the signal frequency to be handled becomes higher, there is now increasingly required not to employ the magnetization mode of axis of easy magnetization but to employ the magnetization mode of hard axis.
When a magnetic film is desired to employed under a high frequency, it is required to provide the magnetic film with a magnetic anisotropy so as to avoid a magnetic resonance such as ferromagnetic resonance and to obtain stable frequency characteristics. An amorphous magnetic material and a metallic magnetic film which exhibits structural relaxation at the occasion of heat treatment can be provided with uni-axial anisotropy in any optional direction by heat-treating them after the formation of magnetic film. With a magnetic device using these materials, it is possible to realize an excellent magnetic characteristic exhibiting minimal loss in a magnetic rotational mode by providing the magnetic film with the uni-axial anisotropy, and subsequently performing the magnetization of hard axis.
However, it is difficult in the case of an oxide magnetic material such as ferrite to provide the magnetic material with magnetic anisotropy by means of heat treatment after the formation of a magnetic film. Accordingly, there is a problem that the magnetic material cannot be used in an MHz (mega Hertz) band since the L value thereof is greatly influenced by the frequency to be employed and the resonance frequency thereof is relatively low.
In an attempt to overcome this problem, there has been proposed a structure as shown in FIG. 1 wherein an insulating body 3 is formed on the surface of a substrate 2 in such a manner that a projected stripe pattern is formed on the surface of the insulating body 3, and a magnetic film 1 is formed on the recessed and projected surface of the insulating body 3. It is possible with this structure to provide the magnetic film 1 with magnetic anisotropy in a specific direction on the substrate 1 (Japanese Patent Unexamined Publication S/56-169309).
However, this proposed structure inherently does not take into account the configuration of magnetic film to be formed with a ferrite material by means of a printing method, which is featured in that the resultant magnetic film is continuous thus making the upper surface thereof approximately flat. Therefore, there is still a problem that it is impossible to provide the flat surface of magnetic film with anisotropy in a desired direction.
There is a problem when a magnetic body is formed into a stripe pattern that the dimension of the pattern is extremely restricted. Further, the easy direction of magnetization prevails in the region where the magnetic film is connected with the coil exhibits, but the hard direction of magnetization prevails in another region, so that it has been impossible to obtain a sufficient effect of anisotropy, thus failing to sufficiently dissolve the problem of frequency dependency of the L value.
As explained above, since it is impossible to employ a ferrite magnetic film having a magnetic anisotropy in a desired direction, the planar magnetic device having a sufficient effect of anisotropy cannot be obtained and hence it has been difficult to employ the planar magnetic device at a high frequency region. On the other hand, in the case where a metallic magnetic film or an amorphous magnetic film is to be employed, it is difficult to sufficiently increase the film thickness, and to avoid a magnetic saturation, thus restricting the electric current to be passed to a magnetic device.